You can compare the hash saved in the database against the text in the password field. If you haven't touch the password (changed the password field), the hashes should match. Let's say your saved hash is xyz, and the hash loaded in the password hash is still untouched, xyz; in this case you don't have to rehash anything, and the hashes should remain the same.

In the other case, let's say your saved hash is xyz, but you edited the password html field to be abc; in this case you will have to rehash the new password (abc) and then replace the old one you have in the database record.